Application Guidelines for MEMS

Compass

Application

RtM

Version 1.3

Agenda 2

Educational part: What is compass? How magnetic compass works, MEMS Magnetometer used as a compass …

ST MEMS Magnetometers: Overview of the key product features

Hardware Design: Schematic, PCB design …

Embedded Software: Tilt-compensated compass, Calibration and Testing

Documentation & Support Tools

AMS Application RtM 07/06/2013

What is compass? 3

AMS Application RtM

• Compass is a navigation instrument that measures direction relative to Earth surface

• There are 4 main directions: north, south, east and west

• Principle of operation • Magnetic compass – based on Earth’s magnetic field

• Gyro compass – based on rotation of the Earth

• Traditional types of compass • Conventional magnetic compass – uses magnetized pointer

• Gyro compass – uses rapidly spinning wheel

• Dry compass – used by mariners in past

• New types of compass • Liquid compass – uses liquid to limit swing and wear out

• GPS compass – based on information from GPS satellites

• Solid state compass – based on magnetic field sensors e.g. MEMS sensors

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LSM303D Applications [1/2]

Where compass is used … 4

AMS Application RtM

• Consumer

• E-compass application in hand held devices (Mobile phones, Tablets, Watches, …)

• Remote controllers for TVs, STBs (Scrolling in the menu, 3D pointer, …)

• Enhanced pointing

• Gaming devices (Accessories for PCs, Tablets)

• Gesture Monitoring

• Key parameters:

• Price

• Size 3x3mm or smaller

• Resolution at least 8 mGauss

• Power consumption 350uA

• Acc. 50Hz ODR + Magn. 6Hz ODR

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LSM303D Applications [2/2]

Where compass is used … 5

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• Navigation • Monitoring and controlling movement of vehical, hand-held device, craft or a person from

one place to another • Portable and fixed navigation systems

• Dead reckoning

• Movement and Position detection • People, Animals and Goods Monitoring (Elderly people monitoring, cow tracking, …)

• Device special orientation

• Others • Mining - Finding direction underground, determining tunnelling

• Astronomy - Compass used for establishing a local meridian

• Building orientation – while Building churches and other houses in a prefered direction

• Key parameters: • Resolution – lower is better (down to 1 mGauss required)

• Size (not so critical as for consumer applications)

• Power consumption – lower is better. Acc. 50Hz ODR + Magn. 6Hz ODR goal is 200uA

E-compass is crucial component for Sensor Fusion Algorithms.

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Magnetic Field 6

AMS Application RtM

• Magnetic field is mathematical description of magnetic influence of

electric currents and magnetic materials.

• It is specified by vector - direction and magnitude (strength). In

literature it is denoted as B or H.

• Units

• tesla (T) is SI unit

• non-SI unit: gauss (G)

1 T =10 000 G

• The Earth's magnetic field is about 0.2 to 0.6 G and has a

component parallel to the Earth's surface that always points toward

the magnetic north pole.

Src: http://www.wikipedia.org

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How a magnetic compass works 7

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• Compass based on measurement of Earth’s magnetic field points to

“magnetic north” – the North magnetic pole of the Earth.

• In navigation, maps and directions are related to geographical or

“true north” – the Geographical North Pole.

• Magnetic north and true north poles are not at the same location. The

angle between directions to the two poles is called magnetic

declination.

• Magnetic declination is in the range of 20º and can vary widely

depending on where the compass is placed on Earth’s surface. Local

magnetic declination is given on most maps to allow the map to be

orientated with a compass.

• Positions of magnetic poles change over time (i.e. hundreds years ..)

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MEMS magnetometer as a compass [1/2] 8

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• Definition of attitude angles: Roll, Pitch and Heading (Yaw)

• The information shown by a compass is Heading.

• Heading is defined as the angle between the Xb axis and the

magnetic north on the horizontal plane measured in a clockwise

direction when viewing from the top of the device (or aircraft).

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MEMS magnetometer as a compass [2/2] 9

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• Local Earth magnetic field H has a fixed component Hh on the

horizontal plane pointing to the Earth’s magnetic north.

• This component can be measured by the MEMS magnetic sensor

sensing axes as Xh and Yh. Then the heading angle is calculated as:

Heading = arctan(Yh / Xh )

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Tilt Compensation using Accelerometer 10

AMS Application RtM

• If a device is tilted, then the pitch and roll angles are not equal to 0 .

• The magnetic sensor measurements XM, YM, and ZM need to be compensated to obtain Xh

and Yh by using accelerometer data.

Xh = XM • cos(Pitch) + ZM • sin(Pitch)

Yh = XM • sin(Roll) • sin(Pitch) + YM • cos(Roll) - ZM • sin(Roll) • cos(Pitch)

• Compensation using accelerometer works well for tilt in +/- 50 range.

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Key parameters for MEMS compass 11

AMS Application RtM

• Magnetometer range: smaller – better, +/- 1 Gauss or higher

• Magnetometer resolution: higher – better

• The device should embed accelerometer for tilt compensation

• Accelerometer resolution: 2mg or better to achieve 0.2deg precision

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LSM303D Key Features Sensor Module: 3-Axis Accelerometer + 3-Axis Magnetometer

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APPLICATION

Compensated Compass Location Based Services Map Rotation Personal Navigation

KEY FEATURES

3-axis accelerometer and 3-axis magnetometer ±2/±4/±8/±12 gauss magnetic field full-scale ±2/±4/±8/±16 g accelerometer full-scale Low accelerometer noise 150 ug/√Hz Low magnetometer noise 5 mgauss/RMS Analog supply voltage: 2.16 V to 3.6 V Digital supply voltage IOs: 1.71 V to Vdd Current consumption 300uA (A+M) 16-bit data out, FIFO for accelerometer Programmable interrupt generators for free-fall,

motion detection and magnetic field detection Automatic Set/Reset internal functionality to cancel

magnetic interference offset 3x3 mm LGA package

KEY ADVANTAGES

High performance g-sensor with antialiasing filter embedded

Offset bridge compensation of the magnetometer Compensation of the sensitivity drift over

temperature for magnetometer Low noise, low current consumption

Building MEMS Compass Process 13

AMS Application RtM

1. Hardware design to make sure the MCU can get clean raw data from the accelerometer and the magnetic sensor.

2. Accelerometer calibration to obtain parameters to convert accelerometer raw data to normalized values for pitch and roll calculation

3. Magnetic sensor calibration to obtain parameters to convert magnetic sensor raw data to normalized values for the heading calculation

4. MCU running heading computation software.

5. Test the performance of the electronic compass system.

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Hardware Design 14

Schematic aspects

PCB design

AMS Application RtM 07/06/2013

Schematic Aspects • Power supply

• Separated VDD and VDD_IO lines (ultra low drop, low noise LDO)

• Supply voltage range: 2.16 to 3.6V for VDD, VDD_IO 1.71 up to VDD

• 1 or 2 digital serial interfaces are used by ST 6D modules with magnetometer • LSM303D:

• SPI 3-wire(CS, SPC, SDI/O) or 4-wire (CS, SPC, SDI, SDO)

• I2C (SCL, SSA) with slave address selectable by SA0 pin

• CS pin is used to select between SPI and I2C

• LSM303DLxx

• I2C interface only

• Device setup and data acquisition is done by accessing registers

• INT1 and INT2 interrupts push-pull pins have programmable functionality

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LSM303x STM8 / STM32

SPI or I2C

INT1

INT2

AMS Application RtM

LDO (LDS3985M30R)

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PCB Design [1/2] Compass placement

• Compass must be located in a magnetically quiet location, far

away from sources that could distort the Earth’s magnetic field from

flowing cleanly through the circuitry:

• Magnets

• Speakers

• Motors

• Steel or ferrous metal shields

• Batteries

• Surface-mount electronic components containing the ferrous metal nickel: leave a

couple od millimeters space between the nickel bearing components and the

compass

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AMS Application RtM 07/06/2013

PCB Design [2/2] High Current Wiring Effect on Compass

• High currents in wiring and printed circuit traces can be culprits in

causing errors in magnetic field measurements for compassing.

• The magnetic sensors can not discern between Earth’s magnetic

field and adjacent.

• Conductor generated magnetic fields will add to Earth’s magnetic field

making errors in compass heading computation.

• Keep currents higher than 10 mili-amperes a few millimeters further

away from the sensor IC.

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AMS Application RtM 07/06/2013

Embedded Software 18

Accelerometer Calibration

Magnetometer Calibration

E-compass Application: Heading computation

Testing the MEMS Compass: Verification of the calibration

AMS Application RtM 07/06/2013

Accelerometer Calibration [1/2]

• All ST MEMS accelerometers are factory calibrated – it is sufficient

for most of the applications

• To reach a heading accuracy of below 2 , an easy calibration

procedure is hereafter described.

• After the LSM303x is installed on PCB inside a device, it is

necessary to calibrate the accelerometer part again by device's

manufacturers in order to determine the offset, the scale factor,

and the misalignment matrix with respect to the device body axes

Xb/Yb/Zb.

• After the device is released to the market, end users don't need to

perform further accelerometer calibration in field.

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AMS Application RtM 07/06/2013

Accelerometer Calibration [2/2]

• Relationship between the normalized Ax1, Ay1, and Az1 and the accelerometer raw measurements Ax, Ay, and Az:

• where [A_m] is a 3x3 misalignment matrix between the accelerometer sensing axes and the device body axes; A_SCi (i = x, y, z) is the scale factor and A_OSi is the offset

• The goal of the accelerometer calibration is to determine the12 ACCxx parameters.

• The calibration can be performed at 6 stationary positions:

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AMS Application RtM 07/06/2013

Magnetometer Calibration Needs

• All ST magnetometers are calibrated in ST fab.

• If no ferrous object are on the PCB and other SMT devices are sufficiently distant from the magnetometer, no calibration is necessary. This is not the case usually!

• If ferrous object are close to the magnetometer an Hard-Iron calibration is necessary (rotation on the flat plane).

• If SMT devices are close to magnetometer an Soft-Iron calibration is necessary (pitch, roll and yaw rotations).

• Definition of terms

• Hard-Iron magnetic materials – ferromagnetic materials with permanent magnetic fields (e.g. magnets, speakers). They are time invariant.

• Soft-Iron magnetic materials – the items (e.g. current carrying traces on the PCB, steel shields, batteries or other magnetically soft materials) which can become magnetized and produce time varying magnetic field.

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AMS Application RtM 07/06/2013

Magnetometer Calibration [1/4]

• The relationship between the normalized Mx1, My1, and Mz1 and the magnetic sensor raw measurements Mx, My, and Mz can be expressed as

• where [M_m] is a 3x3 misalignment matrix between the magnetic sensor sensing axes and the device body axes; M_SCi (i = x, y, z) is the scale factor and M_OSi is the offset caused by hard-iron distortion; [M_si] is a 3x3 matrix caused by soft-iron distortion.

• The goal of the magnetic sensor calibration is to determine the MRxx parameters.

• There are 3 steps for magnetic sensor calibration.

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AMS Application RtM 07/06/2013

Magnetometer Calibration [2/4] Step 1: Soft-iron effect verification

• It is always good to know if the device has soft-iron interference

before choosing which model for the identification of the

calibration parameters, tilted ellipsoid, or non-tilted ellipsoid. This can

be done by performing 3D rotations in a clean environmental area.

• An amount of 3D rotations data can be used for rough field calibration.

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AMS Application RtM 07/06/2013

Magnetometer Calibration [3/4] Step 2: Hard-iron, soft-iron and scale factor

compensation • If there is soft-iron distortion, the 3D rotations show a tilt ellipsoid which can be

described as the following equation:

• where: x0, y0, z0 are the offsets M_OSi (i = x, y, z) caused by hard-iron distortion; x, y, z are magnetic sensor raw data Mx, My and Mz; a, b, c are the semi-axes lengths; d, e, f are cross axis effect to make the ellipsoid tilted; R is a constant of the Earth’s magnetic field strength. Least square method based algorithm.

• If there is no soft-iron distortion inside the device, or the soft-iron effect is very small and can be ignored, then the ellipsoid from 3D rotations is not tilted. So the soft-iron matrix [M_si] is a 3x3 identity matrix and equation above can be simplified as:

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After soft-iron, hard-iron and scale compensations

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Magnetometer Calibration [4/4] Step 3: Misalignment error compensation

• Misalignment error compensation is to align the magnetic sensor sensing axes to the device body axes based on three 2D full round rotations. The vector dot-product method can be used to find each normalized vector that rotates, corrected, three 2D full round rotation circles to their corresponding body axes. The normalized vector means the magnitude is equal to 1.

• Applying the [M_m] 3x3 misalignment matrix to the above unit sphere and three 2D circles, the plot is shown below. Now three 2D full round rotations are aligned to the device body axes. For example, the red color Zb down rotation is parallel to Xb - Yb plane.

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AMS Application RtM After misalignment compensation

07/06/2013

E-Compass Application - Heading computation

For the heading calculation, 3-axis magnetic sensor measurements need to be

normalized by applying magnetic sensor calibration parameters and then reflected

onto the horizontal plane by tilt compensation:

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AMS Application RtM

where Mx1, My1, and Mz1 are the normalized magnetic sensor

measurements after applying calibration parameters

where Ax1, Ay1, and Az1 are the normalized values after applying

accelerometer calibration parameters into Ax, Ay, and Az raw data

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Testing the MEMS Compass

• After the calibration parameters for the accelerometer and the

magnetic sensor of the LSM303D have been determined, it is

necessary to check the performance of the electronic compass.

This could be carried out with accurate lab testing and rough field

testing.

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AMS Application RtM

Lab testing • Convenient setup for accurate lab testing is a

wooden platform with 3 degrees of rotation freedom.

• Rotate the wooden platform horizontally clockwise

or counterclockwise at a random angle which can be

read from the marks on the platform.

• Then compare the compass heading output with the

known heading angle.

Field testing • In any physical situations outside the lab, rough field

testing can be performed.

• A wooden table with a smooth surface is required.

The surface does not have to be leveled.

• Draw some lines, for example, 20º apart on a white

sheet of paper as shown.

• Align the left edge of the device to any line.

• Record the compass heading output value.

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Documentation & Support Tools 28

Datasheet, Application / Design Notes & Tips

Evaluation Boards

PC Graphical User Interface

Technical Support

AMS Application RtM 07/06/2013

ST E-compass SW Libraries • ST has different SW solutions for tilt compensation and Magnetometer

calibration for e-compass

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Version Features Required

Documentation

Tilt

Compensation

Library

• Needs Accel + Magnetometer data Evaluation Agreement -

LUA

Basic Calibration

Library

• Needs only Magnetometer data

• Very light at the computational point of view

Evaluation Agreement -

LUA

iNEMO Engine

Calibration Lite

• Needs only Magnetometer data

• Needs less computation power

• Calculates HI and SI corrections

• It supports two functional mode:

1. Always on calibration (Background Calibration)

2. Triggered Calibration

Evaluation Agreement -

LUA

iNEMO Engine

Calibration

(PRO version)

• Needs complete 9-axis data to Sensor Fusion

• Has better performances but needs more resources

• Uses Kalman filter theory to determine Offsets and “Gains”

• Supports two functional mode:

1. Always on Calibration (Background Calibration)

2. Triggered Calibration

Evaluation Agreement -

LUA

End User Certificate -

EUC

AMS Application RtM 07/06/2013

Documentation

• ST Compasses Product Website

• Application Note AN3192 for tilt-

compensated compass implementation

• Technical Note TN0018 on PCB design

and package surface mounting

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AMS Application RtM 07/06/2013

Evaluation boards Daughter boards available:

STEVAL-MKI109V2

STM32-based MEMS motherboard

compatible with ST MEMS adapters

LSM303D

STEVAL-MKI133V1

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LSM303DLHC

STEVAL-MKI106V1

Note: Schematics and Gerber files are available

under evaluation boards webpages in internet

AMS Application RtM

• Firmware upgrades are possible via DFU

• Source codes available including low

level drivers for STM32

07/06/2013

STM32F3 – Discovery Kit 32

• New Discovery STM32 (M4) kit which

embed 9-axis sensors:

• LSM303DLHC

• L3GD20

• Including source codes for a compass.

AMS Application RtM

STM32F3DISCOVERY Web Click to download

07/06/2013

Unico Evaluation software

• Unico is Graphical User Interface (GUI) for PC (Windows based)

• Designated to be used with STEVAL-MKI109V2 and any MEMS

adapter board

• Connection

• USB

• Bluetooth – with STEVAL-MKI132V1

• Compass in Unico

• Register setup

• Accelerometer and magnetometer data reading

• Compass implemented in PC GUI

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AMS Application RtM

SOFTWARE PACKAGE Click to download

07/06/2013

Analog, MEMS & Sensors (AMS)

Application Support Team

… and RF IPD/IPAD RF Baluns

… is providing technical application

support for customers, designing in ST

Analog, MEMS & Sensors products, in

projects agreed with local EMEA ST

sales office / Technical marketing team

• Solving

• Product and Application problems –

answering detailed technical questions

• Providing

• Design consulting (Schematic, PCB and

Software)

• Technical Trainings

Contact email: AMS-support-EMEA@st.com

Application Support Service Card

Application Support Team Focus

AMS Application RtM 07/06/2013